1. Field of the Invention
The present invention relates to semiconductor wafer handling and processing equipment, and in particular, to a device for identifying a center of a wafer and reading an OCR mark on the wafer, wherein the device includes a buffer mechanism that allows a high throughput of wafers through the device.
2. Description of the Related Art
During the fabrication of semiconductor wafers, wafers are transported between the various process tools in the wafer fab in open cassettes or cassettes sealed within a pod such as a standard mechanical interface (SMIF) pod. As wafers move through the various processing tools, it is desirable to be able to track and locate a particular wafer at any given time. Moreover, it is desirable to be able to identify a particular wafer during wafer fabrication to ensure that the wafer is subjected only to processes appropriate for that wafer. This wafer tracking is accomplished in part by marking each wafer with an optical character recognition (OCR) mark, or similar indicial mark, which mark is read for each wafer prior to locating a wafer within a processing station. The indicial mark is typically a number or letter sequence etched into an upper and/or lower surface of a wafer near the outer circumference by a laser or other suitable etching means. The indicial mark may alternatively be a bar code or a two dimensional dot matrix at an outer circumference of the wafer.
Before an indicial mark may be read, the mark must first be located. When a wafer is seated within a wafer cassette, the orientation of the wafer to the cassette, and to a tool for extracting and supporting the wafer, is generally unknown. Attempts have been made to align the indicial mark of each wafer at a particular rotational orientation within the cassette. However, because wafers move within a cassette upon handling and transfer of the cassette between processing stations, and because the wafer is often reoriented in a process tool, alignment of the indicial marks prior to transportation has not proved feasible.
In order to locate an indicial mark, wafers are conventionally formed with a fiducial mark, such as a notch or flat, on the outer edge of a wafer. For each wafer being processed, the indicial mark is located in a fixed, known relation to the notch, and by finding the notch, the precise location of the indicial mark may be determined. The notch is typically found by rotating the wafer under a sensor or camera so that the sensor or camera scans the outer edge of the wafer and identifies the radial position of the notch. Once the notch location is found, the wafer can then be rotated to position the indicial mark under a camera (the same or different than that used to find the notch), and the indicial mark may then be read.
Another reason for rotating the wafer is to determine the radial runout of the wafer. It is important that a wafer be centered when seated in the cassette or on a process tool. If a wafer is off center, it may not properly seat on a chuck of a process tool, and/or it may generate particulates by scraping against the sides of the cassette upon transfer of the wafer to the cassette. Therefore, it is desirable to determine the radial runout of the wafer and correct it to the center position. The radial runout is a vector quantity representing the magnitude and direction by which a wafer deviates from a centered position with respect to a tool on which the wafer is seated. Once the radial runout has been determined, the wafer may be moved to a center position, or the end effector of the robot may shift to acquire the wafer on center.
Conventionally, a separate operation has been devoted to locating and reading the indicial mark on the wafer, and determining and adjusting for the radial runout of the wafer, at or immediately prior to each station where it is desired to identify the particular wafers to be processed in that station. FIG. 1 shows a conventional station 20, including a module 42, typically referred to as an aligner, for performing the operation of wafer centering, notch orientation, and indicial mark reading. Such stations are conventionally located immediately upstream, as a stand alone unit, or as part of each processing station in the wafer fabrication process where the indicial mark is to be read. In addition to aligner 42, the station 20 includes a wafer handling robot 22 for accessing and transferring wafers 40 from a cassette 38. The robot 22 includes an end effector 32, and is controlled by a computer 36 such that end effector 32 transfers the wafers 40 between the cassette and the aligner 42.
The aligner 42 includes a chuck 44 capable of rotation. The robot 22 deposits the wafer on chuck 44, and the chuck then rotates the wafer to identify the location of the indicial mark and to determine the radial runout. Various digital sensors are known, such as for example digital sensor 48, for identifying the notch and determining the radial runout of the wafer. Once the position of the notch has been identified, the indicial mark may thereafter be positioned under and read by a video camera 46 also mounted on the aligner 42.
In conventional aligners, the robot must first transfer the wafer from the cassette to the aligner, the aligner then identifies the radial runout and notch position, the robot or aligner centers the wafer, and then the robot transfers the wafer back to the cassette. The robot sits idle while the aligner performs its operations, and the aligner sits idle while the robot transfers the wafers to and from the aligner. Conventional aligner/robot systems therefore have a relatively low throughput, on the order of approximately 300 wafers per hour. This low throughput is significant as the alignment process must be performed at each station where an indicial mark reading is required, and must be performed on each individual wafer at each of these stations. It is known to provide dual armed robots to increase throughput. However, such robots take up additional space within the tool and cleanroom, where space is at a premium. Moreover, the dual armed robots are expensive, require more complicated controls, and are relatively difficult to maintain.
It is therefore an advantage of the present invention to increase the throughput of wafers processed by an aligner.
It is another advantage of the present invention that the aligner does not sit idle while the robot transfers wafers to and from the aligner.
It is another advantage of the present invention that the robot does not sit idle while the aligner performs its operations.
It is a further advantage of the present invention that it does not take up additional space within the cleanroom.
It is a still further advantage of the present invention that a second wafer may be loaded onto the aligner before processing of a first wafer is complete.
It is another advantage of the present invention that conventional aligners may be easily modified at a small cost to operate in accordance with the principles of the present invention.
It is a still further advantage of the present invention to provide an improved sensor system for identifying wafer runout.
It is another advantage of the present invention that it is more economical than convention high throughput systems such as those employing duel end effector robots.
These and other advantages are provided by the present invention which in preferred embodiments relates to an aligner including a buffer mechanism having a buffer paddle on which wafers may be buffered to increase the throughput of the aligner and the system in general. The aligner according to the present invention may be seated within a minienvironment affixed to or part of a process tool, or may alternatively be provided as a standalone unit separate and apart from a process tool. In addition to the aligner, the system includes a wafer handling robot. Once a wafer-carrying cassette is loaded into the minienvironment, the robot is capable of transferring the wafers between the cassette and the aligner. The buffer mechanism allows the robot to bring a second wafer to the aligner while a first wafer is processed, and then allows the robot to carry the first wafer away from the aligner while the second wafer is processed. Thus, the aligner does not sit idle while the robot transfers wafers to and from the aligner, and the robot does not sit idle while the aligner performs its operations.
The aligner is provided in general to identify the location of a fiducial mark, such as a notch, position the associated indicial mark for reading by a camera, and determine the radial runout of the wafer. The aligner includes a rotating support platform in the form of a chuck on which the wafers are received from the robot. A motor rotates the chuck so that the radial runout and notch of the wafer may be identified. According to the present invention, the aligner further includes a buffer mechanism having a buffer paddle and a drive mechanism for vertically translating the buffer paddle. The aligner further includes an analog sensor for determining the radial runout of, and/or for identifying the position of the notch and indicial mark on, a wafer being rotated on the chuck.
In operation, a wafer is first loaded onto the chuck by the robot from the wafer cassette. After the position of the notch is identified, the indicial mark positioned and read, and the radial runout determined, the buffer paddle lifts the first wafer off the chuck. While these operations at the aligner are taking place, the robot returns to the cassette and acquires a second wafer and returns to the aligner. Thereafter, the robot deposits the second wafer on the chuck for processing, and acquires the first wafer from the buffer paddle in a centered position. The robot then returns the first wafer to the cassette, or transfers the wafer to a process tool.
After the first wafer has been carried away from the aligner, and while processing on the second wafer is taking place, the robot acquires a third wafer from the wafer cassette and returns to the aligner. At this point, the buffer paddle is positioned above the second wafer seated on the chuck. The end effector deposits the third wafer on the buffer paddle, acquires the second wafer from the chuck and removes it from the aligner. Thereafter, the buffer paddle with the third wafer thereon moves downward to deposit the third wafer on the chuck, whereupon the position of the notch is identified, the indicial mark is read, and the radial runout is determined. The aligner cycles through the above steps until each of the wafers in the cassette have been processed.